This invention relates to novel printing plates, to a method for their preparation and to a lithographic printing process employing the plates.
Printing plates suitable for offset lithographic printing are known which comprise a support having non-image areas which are hydrophilic and image areas which are hydrophobic and ink-receptive.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Ink-jetting is the non-impact method for producing images by the deposition of ink droplets on a substrate in response to digital signals.
JP-A-53015905 describes the preparation of a printing plate by ink-jetting an alcohol-soluble resin in an organic solvent onto an aluminum printing plate.
JP-A-56105960 describes the formation of a printing plate by ink-jetting onto a support e.g. an anodised aluminum plate an ink capable of forming an oleophilic image and containing a hardening substance such as epoxy-soybean oil together with benzoyl peroxide or a photohardening substance such as an unsaturated polyester.
European Patent Application No 0882584 describes a method of preparing a printing plate comprising producing an oleophilic image on the surface of a support by ink-jet printing the image on the surface using an aqueous solution or aqueous colloidal dispersion of a salt of a hydrophobic organic acid e.g. oleic acid.
GB Patent Application No. 2,332,646 describes a method of preparing a printing plate comprising producing an oleophilic image on the surface using an aqueous solution or colloidal dispersion of a polymer bearing water solubilising groups wherein the water-solubilising groups interact with the surface of the support thereby binding the polymer to the support and rendering the polymer insoluble. In the method described the polymer containing water solubilising groups is dispersed in water to form the solution or emulsion.
Long press runs with plates made by jetting a fluid onto the ink accepting image areas of the plate require that the fluid harden or cross-link into a layer which will not wear off under the conditions of the lithographic
A method of preparing printing plates using ink-jetting is required which avoids the use of organic solvents and/or light sensitive materials.
The present invention provides a solution to these problems by a method which employs an aqueous emulsion of an organic polymer prepared by emulsion polymerisation and which is applied to the plate and caused to coalesce.
According to the present invention there is provided a method for the preparation of a lithographic printing plate which method comprises forming an oleophilic image on the surface of a hydrophilic support by depositing, preferably by ink-jetting, the desired image on the surface using an aqueous emulsion of an organic polymer prepared by emulsion polymerisation wherein the polymer is film-forming and adheres to the surface of the printing plate forming an oleophilic film.